Gonna second the method
@anime mentioned. I haven't come across anything else (yet) that could outpace the Mericarol kill rate this method opens up.
Some pointers for running as a RA (IMO, ideally, a Cast RA):
- Get yourself an S-Rank Needle with Hell (and pair it with a V502 for best results) as this will allow you to 'erase' the 4 spawns that aren't Mericarols quickly.
- For the Mericarols themselves: a Frozen Shooter paired with either a Heaven Striker (ideally with A. Best and maybe some Hit) or some Yasminkov 9000Ms will do nicely for dispatching.
I would strongly encourage you to hold off on starting these runs until you can fulfill the above pointers. As a general rule of thumb I would apply to any uber rare hunt: make sure you have the best loadout for it possible with a character that is not only ideal (or at least effective) but also has core stats at or near max. Remember, this is something you're gonna be running for a very long time, and very likely to repeat the run several thousand times. Being able to clear each run 10-30 seconds faster will add up exponentially over time.
Oh, and I would also recommend doing your runs only during DAR and RDR boost weeks. During either period, the drop rate is reduced from 1/12609 to an approximate 1/10K. Assuming the adjusted drop rate is 1/10K and you're clearing 3 Mericarols every 3 minutes (or less, just a conservative estimate), you can statiscally expect to do around 3,300 runs across 160+ hours of play time.
Good luck, and may RNGeezus smile upon you-- for he has forsaken me.